Coliphage P1 mediates generalized transduction and also has specialized transducing derivatives (P1std). In the prophage form it is a stringently controlled supercoiled plasmid, while bacteriophage P1 populations have permuted DNAs. Many of the P1std have IS1 at the junction of their P1 and foreign gene segments. One goal of the research program is the further description of lytic cycle chromosomal forms inclucing: origin(s) of chromosomal replication; direction(s) of replication; and the size and topology of lytic cycle chromosomal intermediates. A second goal is a further elucidation of processes of P1std formation, focusing on IS1 functions. P1 mutant for P1std formative processes have been isolated and will be characterized by genetic and biochemical methods.